The dimensioning of a rotary electric machine depends on its nominal torque. The greater the torque that a motor is capable of delivering, the more bulky the electric motor, all other things being equal. There are, however, applications for which it is desirable to obtain both high powers and a great motor compactness. To give a simple physical example, when electric drive motors are to be installed in the wheels of motor vehicles, it is desirable to be able to develop powers of at least 10 kW per motor, and even mostly at least 25 or 30 kW per motor, for the lowest possible weight in order to limit as much as possible the unsuspended weights. It is also desirable for the bulk to be as small as possible, exceeding by as little as possible the internal volume of the wheel so as not to interfere with the elements of the vehicle in the movements of the suspension and in other types of movement of the wheel relative to the body shell of the vehicle.
These two imperatives (high power, low bulk and weight) make the installation of electric drive motors in the wheels of private passenger vehicles highly problematical, unless there is a radical improvement in the power/weight ratio of the electric machines currently available on the market.
The choice of a high speed for an electric motor when designing the motor is one solution that makes it possible, with a given power, to reduce the torque, and therefore the bulk. In other words, for a given nominal motor power, the greater its nominal rotation speed, the smaller its bulk.
A specific design for obtaining high rotation speeds has already been proposed in the patent application EP 1001507. The speeds targeted in this patent application are of the order of 12 000 rpm, by proposing for this a particular arrangement of the assembly consisting of a polygonal amagnetic single-piece shaft and pole pieces shrewdly positioned around this shaft. A refinement that makes it possible to target speeds of the order of 20 000 rpm has been proposed in the patent application EP 1359657, by proposing for this an arrangement using wedges to radially block the magnets in their recesses between the pole pieces.
The pole pieces consist of a stack of metal sheets, each metal sheet being cut in the form of a disc segment. The method for manufacturing these pole pieces and assembling the rotor is particularly lengthy and complex in order in particular to ensure the best alignment of the metal sheets in the stack.